STRUCTURAL GEOLOGY, TECTONIC GEOMORPHOLOGY, AND RECENT SLIP HISTORY OF THE SOUTH-CENTRAL SAN ANDREAS FAULT

Discontinuities are a fundamental characteristic of fault zones and they strongly influence rupture dynamics at seismological time scales and the strength variation over the earthquake cycle and longer time scales. How discontinuous are the faults that define the fault zone? On what spatial scales are they discontinuous? Is the zone so discontinuous that it can be treated as effectively a continuous shear zone? Over what time scales do the discontinuities develop? Integration of geologic and geomorphic mapping and data about recent and prehistoric earthquakes along the Parkfield to Carrizo segments of the San Andreas Fault (SAF) provide the data to evaluate these questions. Geologic and geomorphic mapping near SAFOD at Middle Mountain show a history of slip along fault surfaces across a several km-wide zone. A granite block exposed several km southeast of SAFOD is several hundred m long, a few tens of m wide, and tapers vertically in tens of meters. This hull-shaped block and numerous others of similar and smaller dimensions provide examples of fault block form. Ground cracking in the 1966 and 2004 Parkfield earthquakes was not along the same fault trace but stepped several tens of meters to the southwest. Recent imaging of the landscape along the SAF with airborne laser swath mapping data ("B4 project”), provides a new quantification of the topography and enhances our ability to decipher the motion histories of fault blocks. The most recent slip is well localized along single fault traces continuous for hundreds to thousands of meters. Slip along these traces can be measured confidently using the B4 data and attributed to the last several earthquakes along the SAF. Previously active traces are still evident in the geomorphology. Tilts and uplift histories of the fault blocks are indicated by drainage network responses. Blocks with aspect ratios of ~10:1 (parallel:perpendicular to the fault) are bounded by variably active fault surfaces and have vertical displacement rates as high as a few mm/yr. These observations and inferences suggest a model in which the upper few km of the SAF zone is comprised of relatively independently moving moderate aspect ratio blocks elongate parallel to the SAF. Their relative motions are controlled by fault activity which may vary on the earthquake recurrence timescale.